Repository deletion SR no longer accepted when gitea backing in place

Observed:

```
$ osc dr network:dhcp -r 15.3 -m 'remove EOL distro'
Server returned an error: HTTP Error 403: Forbidden
The target project network:dhcp is managed in an external SCM: https://src.opensuse.org/dhcp/_ObsPrj.git#master
```

Expected:

Succeed. `osc meta prj` data is not tracked in gitea as far as I can see.

cc @AdamMajer